Upgraded to a Pertronix electronic ignition a couple months ago and put in some AC Delco R46S that I bought from ebay but lately I've been experiencing some misfire and rough idle/engine shake at idle. I came to the conclusion I just dont trust the old plugs. Going to be replacing them with either Autolite 86 or NGK but need to know the proper gap. I fished through threads and some recommended keeping it at stock .35 and some say take it up to .45. It has a Mallory high output ignition coil. I will most likely be going with the Autolite 86 plugs for cost efficiency. Suggestions? Thanks

 

I'd start the new plugs at .040 and see how that works. A gap of .35 or .45??? I don't think so. Just messing with you. I know you meant .035 , .045

The 80's and 90's HEI's would run gaps of .050 - .080 depending on year. I don't think you want to go that high.

The rough idle and shake doesn't sound spark related, suggest you check your timing and fuel mixture settings

 

I agree with Allan. Pertronix suggested with my unit to go up to .035 from .030 gap for my application. Thats where I am at now and it seems good. I run Autolight 85's. They seem to hold up pretty well.

I haven't had the best of luck with ebay/internet buys related to my car. When I can, I like to have the part in my hand to do the old chinese diagram to see if the peice/part fits my application.

Your problem may be timing related.

 

I'm thinking too lean a mixture to start with